This invention relates to an electrically heated sprue bushing for injection molding, and more particularly to an improved connector assembly therefor and method of making the same.
The applicant's U.S. Pat. No. 4,238,671 entitled "Sprue Bushing With Cast In Heater Element" which issued Dec. 9, 1980 shows a sprue bushing used to convey pressurized melt from a molding machine to the mold. The sprue bushing has an electrical heating element with a helical portion surrounding the runner passage and a lead portion which extends out through an aperture for connection to an external power source. While providing several advantages, this structure has been found to have the disadvantage that the lead portion of the heating element is very susceptible to failure at the point where it passes through the aperture due to its not having sufficient strength to withstand stresses or blows it may receive accidentally. Due to the integral structure of the sprue bushing, this normally cannot be repaired and necessitates the replacement of the whole unit.
More recently, the applicant's U.S. patent application Ser. No. 217,115 filed Dec. 17, 1980 entitled "Sprue Bushing and Method of Manufacture" describes an improved sprue bushing in which this problem is overcome by casting the lead portion of the heater element in copper alloy to be solidly integral with the rest of the unit. While this provides sufficient structural strength to avoid breakage, it has the disadvantage that the copper alloy conducts the heat further out with the result that the connections to the external lead wires are susceptible to over heating.
Therefore, it is desirable to provide a connector assembly structure which has a combination of sufficient structural strength and a facility for making a durable connection to the external lead wires, which may be conveniently manufactured by a method incorporating the previously developed special techniques for providing the rest of the sprue bushing.